7.1 Function and Benefits
Insulating Glass Units (IGUs) are designed to provide thermal insulation for building envelopes. They are used to reduce building heat loss and heat gain depending on the climate and IGU combination. Insulating Glass Units are sometimes called Double Glazing, Double Glass Units (DGU) or Sealed Insulating Glass Units (SIGU).
Traditionally the housing energy efficiency building codes in New Zealand have lacked recognition of the insulation properties of windows, and have concentrated on providing minimum thermal resistance values (R Values) for the roof, floor and walls only.
The new Standards have included glazing and focus on the excessive heat loss through the area of single glazing, which is typically 40 to 45m2 on an average house but can be much larger in modern house designs. Extensive cost benefit analysis has been done to show how the correct insulation of a house can improve the energy efficiency of the house and provide energy savings and improved comfort for both the owner and the country as a whole.
How IGUs Work
Clear monolithic glass accounts for less than 5% of a window’s insulation value, the rest being supplied by the still air layers of the environment on either side of the glass. Since the heat flow resistance of still air is much greater than that of glass, a glass unit made of two panes enclosing an air space will have about twice the insulation value of a single pane window (half the heat loss).
Triple pane units with two sealed airspaces have an insulating value approximately three times that of single glazing. This is why double and triple glazed units are called Insulating Glass Units as they provide insulation to the windows of a building, like fibre-glass insulation provides insulation to the wall.
The Benefits of IGUs
- Reduce heat loss.
- Save energy.
- Reduce condensation.
- Increase comfort.
- Reduce noise.
- Reduce heat gain.
- Reduce glare.
- Reduce fading.
- Increase security.
Insulating Glass Units retain much more heat in a room during winter reducing
heat loss and saving energy. They reduce noise penetration and window condensation and provide
warmer zones near windows to increase comfort.
For summer, with the use of solar control glass, they can reduce heat gain, glare and fading and increase air conditioning efficiency. They are harder to break than single glass and the shards normally stay in place after breakage, increasing security.
Refer to section 11.1.1 an Architectural Selection flow chart and guide to assist in the selection of Insulating Glass Units for windows and doors.
IGU Selection for Colder Climates
The key factors for buildings in colder climates are reducing heat loss to keep the building warm and allowing some solar heat gain to warm the building.
- The performance of an IGU in reducing heat loss can be enhanced by using large airspaces or by using Argon Gas.Larger airspaces up to 16mm provide better insulation than the minimum 6 to 8mm and it is always advisable to achieve 12 mm or wider air spaces. Argon Gas can be used to fill the unit as it provides better insulation than air.
- The performance can be further enhanced with the use of Low E glass as it traps the heat inside the building. The coating is normally used on surface 3 to reflect the building’s long wave radiation and is normally a clear or neutral colour with relatively high light transmission.
- When solar gain is required to heat the building, the glass is usually clear as it allows the maximum solar gain. Low E glasses will slightly reduce the light transmission and can slightly increase the reflectivity of the windows.
- Daylight and glare will be at their maximum with clear glass. Solar control glass and some Low E glasses will reduce the light transmission and control glare.